New study finds Los Angeles and San Diego are at risk from M=7.3+ offshore earthquakes

This picture from the Scripps Institution of Oceanography shows the research vessel New Horizon towing a hydrophone to map the bathymetry of the Newport-Inglewood/Rose Canyon fault zone. (Photo from: Scripps Institution of Oceanography)

A new study by researchers at the Scripps Institution of Oceanography, the Nevada Seismological Laboratory, and USGS has revealed that the Newport-Inglewood/Rose Canyon fault zone, which extends offshore from San Diego to Los Angeles, is capable of rupturing in M=7.3+ earthquakes. Such a quake could cause significant damage to some of the most densely populated areas in California.

The Newport-Inglewood/Rose Canyon fault zone is a strike-slip system in Southern California, and is part of the Pacific-North American plate boundary. Prior to this study, the Newport-Inglewood and Rose Canyon faults were believed to be separate systems. However, using a combination of seismic surveys, and high-resolution bathymetric data, the researchers determined that they are actually a continuous fault system, posing an even greater hazard to millions of people. By using these two types of data, which differ in their resolution and depth of penetration, the team was able to define the fault structure in great detail, which could then be used to calculate earthquake magnitudes.

The Newport-Inglewood/Rose Canyon fault zone exhibits a mainly continuous fault trace over four distinct segments. These segments are separated by what geologists term as stepovers, which are where a fault is horizontally offset. Most scientists believe that stepovers at least 3 km wide are likely to prevent through-going ruptures. However, along the Newport-Inglewood/Rose Canyon fault zone, none of the three stepovers exceed 2 km in width, leading the team to believe that an earthquake could rupture the entire length of the fault zone in M=7.3+ events. This was based historical ruptures worldwide which have been shown to rupture through stepovers of this width.

This Temblor figure shows the Southern California coastline both with and without faults. The offshore Newport-Inglewood/Rose Canyon fault zone poses a significant threat to both Los Angeles and San Diego. The new research shows that M=7.3+ earthquakes are possible along this fault zone. The yellow star in this figure marks the location of the 1933 M=6.4 Long Beach earthquake which killed 113 people.

While the likelihood of a full rupture sits at 30-40%, lead author Valerie Sahakian, who is now a postdoctoral fellow at the USGS in Menlo Park, says that a smaller earthquake in the magnitude 5 or 6 range could still have a significant impact on the region. Such an earthquake happened in 1933, when the M=6.4 Long Beach earthquake killed 113 people.

Overall, this new study provides additional evidence showing the risk offshore faults pose to Southern California. Because the Newport-Inglewood/Rose Canyon fault is never more than four miles from the coast, significant shaking from an earthquake could have great impacts to the cities of Los Angeles and San Diego. Therefore, further research is needed to improve the seismic hazard models for the Southern California coast.